1. Field of the Invention
The present invention relates to inflator propellant compositions adapted for use with compressed gas-based air bag automotive safety systems.
2. Description of Related Art
Current usage of propellants in automotive safety applications falls into at least two classes: those that serve as inflators for air bags (either driver, passengers or side impact) and those that serve to tighten (remove slack from) more conventional restraint systems such as lap/shoulder belts. This invention addresses inflator propellants. Current inflator propellants can be further divided into those for inflators;called pyrotechnic type inflators, where the propellant provides all of the inflation gas, and those for so-called hybrid inflators where the propellant heat energy and gas serves to supplement a pre-pressurized volume of inert gas contained in a pressure bottle so as to speed up the inflation process and reduce the volume and size of the pre-pressurized gas source. This invention further addresses hybrid inflator systems.
Current state-of-the art propellants for pyrotechnic inflators, where all of the gas is provided by the propellant, typically consist of a tableted mixture of sodium azide and an oxidant such as iron oxide. The byproducts of such a reaction are free iron, sodium oxide and gaseous nitrogen. Regardless of the oxidant, solid residue comprises a substantial proportion of these combustion products which must be filtered out of the gas stream via an elaborate filtration system.
The current state-of-the art propellant for hybrid inflator systems is comprised of a mixture of polyvinyl chloride, a suitable plasticizer, and potassium perchlorate as an oxidizer. Although this propellant is suitable in terms of heat output and burning rate, a major combustion product is very finely divided potassium chloride (KCl). The KCl, dispersed as an aerosol, upon exhausting from the gas bag, acts as an obscurant which may impede occupant egress from a vehicle and, if inhaled, can cause reactions in allergic individuals.
The gas used to inflate the gas bag in both pyrotechnic and hybrid inflators must meet stringent requirements regarding toxic components such as carbon monoxide (CO) and oxides of nitrogen (NO.sub.x), as well as thermal stability. These requirements are a crucial factor mandating the propellant types used to generate the inflator gas. Clean burning, conventional smokeless propellants containing nitrocellulose and nitroglycerin are unacceptable for use in either type because of the significant amounts of CO in their product gases when used in pyrotechnic inflators and their poor thermal stability when used in either system.
One type of hybrid inflator utilizes a pre-pressurized mixture of gaseous oxygen (up to 20%) and Argon. Because of the presence of oxygen, this type of hybrid inflator is capable of oxidizing carbon monoxide and hydrogen generated as combustion products in conventional propellants to the more desirable carbon dioxide and water, thus making possible the use of more conventional propellants which are not fully oxygen balanced. Poorly oxygen balanced propellants of the type commonly known as LOVA, based primarily on the energetic ingredient cyclo trimethylene trinitramine (RDX), have been tested and, although they possess the necessary thermal stability, their burning characteristics, i.e., low burning rate and tendency to self-extinguish, make them unsuitable.
The prior art discloses various other propellant compositions for use in automotive safety applications. For example, U.S. Pat. No. 5,125,684 discloses a stable, extrudable, non-azide crash bag propellant composition and a low temperature process for producing the same from an extrudable mass containing an effective amount of a cellulose based binder. The composition comprises 45-80 wt. % oxidizer salt, a cellulose based binder and 10-35 wt. % of an energetic component selected from a group which includes cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX).
EP 591119 A2 discloses a gas emitting substance for inflating an accident safety airbag, comprising one or more high energy explosive(s) including 50.95% of (RDX) and/or (HMX) of a mean particle size of 1-20 microns, up to 5 wt. % of nitrocellulose; and 5-50 wt. % of a combustible, energetic or non-energetic binder, preferably one or more of polyurethane, cellulose acetate butyrate, hydroxy terminated polybutadiene, ethyl cellulose, glycidyl acid polymers and polymers of either 3-nitratemethyl-3-methyl oxymethane or glycidyl nitrate.
Other prior art patents disclose explosive or rocket propellant compositions which are not specifically designed as safety air bag propellants. For example, U.S. Pat. No. 3,954,528 discloses solid gas generating and gun propellant compositions employing triamino-guanidine nitrate as a propellant ingredient in admixture with an oxidant and a compatible synthetic polymer binder material. The oxidant may be selected from cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX).
U.S. Pat. No. 4,689,097 discloses that a mixture of a nitramine and triaminoguanidium nitrate accelerates the burn rate for certain low smoke propellants. The nitramine may be selected from cyclotrimethylenetrinitramine (RDX) and cycloetetramethylenetetranitramine (HMX) or mixtures thereof. Fine triaminoguanidium nitrate particles and coarse nitramine particles are shown to be used in crosslinked propellant compositions.
U.S. Pat. No. 5,061,330 discloses a cast cured propellant and explosive made from a mixture of a polyglycidal azide polymer, an energetic plasticizer such as trimethylolethane trinitrate (TMETN) and HMX or RDX. The composition may also contain aluminum powder.
In addition, U.S. Pat. No. 5,316,600 discloses a castable, energetic, plastic-bonded explosive containing glycidyl azide polymer (GAP) combined with the energetic plasticizers trimethyloethane trinitrate (TMETN) and triethylene glycol dinitrate (TEGDN) or bisdinitropropyl formal and acetal mixture (BDNPF/A), and the explosive solid cyclotetetramethylene tetranitramine (HMX) or cyclotrimethylene trinitramine (RDX).
However, none of these latter references discloses propellants compositions which are stable enough to function satisfactorily in hybrid inflator systems in such a way that the burning rates are increased and their tendency to self-extinguish is reduced.